The invention relates to a method for making high quality InGaAsN semiconductor devices using metal organic chemical vapor deposition.
InGaAsN is an attractive material for a variety of semiconductor applications. InGaAsN is useful in the area of long wavelength edge-emitting lasers and vertical cavity surface emitting lasers (VCSEL) in the optical communications area because the material is readily grown on GaAs wafers which provides higher conduction band offset and allows the use of GaAs/AlAs high reflective distributed Bragg reflectors (DBRs).
A number of research groups have achieved room temperature continuous wave (CW) operation of InGaAsN VCSELs that were made using molecular beam epitaxy (MBE). Room temperature continuous wave operation of InGaAsN VCSELs has been achieved in VCSELs made using metal organic chemical vapor deposition (MOCVD) using dimethylhydrazine (DMHy) as a source of nitrogen. MOCVD is preferred over MBE as the growth technique for achieving mass production. To achieve commercial use of InGaAsN VCSELs, further improvements in device performance such as lowering the threshold current density and extending the device lifetime are necessary.
It has been reported by Sato et al. in Electronics Letters, 33, 1997, 1386, that the surface morphology of the InGaAsN active region grown directly on the AlGaAs cladding layer using MOCVD appears powder-like, indicating three dimensional growth. Kawaguchi et al. report in Electronics Letters 36, 2000, 1776, that continuous MOCVD growth of InGaAsN layers on GaAs/AlGaAs layers results in poor optical quality of the InGaAsN layer while switching to a two reactor process yields a substantially better optical quality InGaAsN layer. Sato et al., IEEE Photonics Technology Letters, 12, 1999, 1386 achieved good results for a highly strained GaInAsN ridge stripe laser using MOCVD by using aluminum free cladding layers. This approach is not useful for VCSELs because the highly reflective AlGaAs/GaAs DBR mirror can not be used.
Investigation by secondary ion mass spectroscopy (SIMS) indicates that conventional metal organic chemical vapor deposition (MOCVD) growth of InGaAsN active layers on GaAs/AlGaAs layers results in appreciable aluminum contamination (close to one percent) and is responsible for performance loss in a variety of semiconductor devices. Poor optical properties of InGaAsN active regions for InGaAsN edge emitters and InGaAsN VCSELs as well as lower than expected current gain in bipolar transistors are attributable to Al contamination of the active regions. In InGaAsN solar cell and InGaAsN photodetector structures (see for example, R. R. King et al, Conference Record of the 28th IEEE Photovoltaic Specialists Conference, 2000, 998 and J. B. Heroux et al, Applied Physics Letters, 75, 1999, 2716), elimination of Al contamination results in high quality InGaAsN absorbing layers that result in higher quantum efficiencies for those semiconductor devices. Additionally, InGaAsN active regions grown over AlGaAs layers show more O and C incorporation than InGaAsN active regions grown without underlying AlGaAs layers. To avoid aluminum incorporation into InGaAsN layers, embodiments in accordance with the invention are disclosed.
In accordance with the invention, either layers are grown in the semiconductor structure whose growth serves to getter Al atoms/Al containing molecules or chemicals may be introduced into the MOCVD chamber via flow gases that serve to getter Al atoms/Al containing molecules. The methods reduce the Al content that is incorporated into the N containing layers and result in improvements in the quality of the InGaAsN layer including smoother surface structure, improved optical qualities and lower levels of recombination centers due to, for example, O and C incorporation that typically accompanies the Al incorporation.